1. Field of the Invention
The present invention relates to in-vacuum lithography. More particularly, it relates to photoresist outgassing in an in-vacuum lithography system.
2. Related Art
One of the many processing steps for manufacturing microelectronic circuits on a semiconductor wafer includes coating the wafer with a thin layer of photoresist and exposing the coated wafer to a source of light through a patterned mask. This process is known as lithography. The size of the microelectronic circuit features that can be produced using lithography is inversely related to the wavelength of the light used to expose the coated wafer.
In order to reproduce very fine microelectronic circuit features, a source of extreme ultraviolet (EUV) light, such as a laser-produced plasma (LPP) or synchrotron, must be used. Using EUV light, it is possible to reproduce microelectronic circuit features down to 0.03 micron. Because EUV light is readily absorbed by matter, EUV lithography is carried out in a vacuum.
One means for performing lithography is described in U.S. Pat. No. 4,408,338 to Grobman (hereinafter Grobman). Grobman describes a form of x-ray lithography known as contact or proximity printing. In contact printing, the wafer to be exposed is placed very close to the mask, and there are no reducing optics used between the mask and the wafer. The features of the mask are reproduced on the wafer without reduction. This aspect of contact printing, however, makes the masks used in contact printing systems both difficult to design and expensive to produce. Furthermore, it makes contact printing impractical for many applications such as, for example, application specific integrated circuits and systems on a chip that have very small circuit features.
In order to reduce the difficulty and costs associated with designing and producing masks for use in an EUV lithography system, it is highly desirable to include projection optics in an EUV lithography system between the mask and the wafer to be exposed. Projection optics can be used to reduce the size of the features reproduced on the wafer, and thereby allow masks to be used that have larger patterns.
It is a property of photoresist that it outgases or produces byproducts, especially when it is exposed to high energy light. These outgassed resist products are generally referred to herein as xe2x80x9cresist gases,xe2x80x9d xe2x80x9cresist outgases,xe2x80x9d or xe2x80x9coutgases.xe2x80x9d Among the outgases produced by photoresist are hydrocarbon molecules that can condense on the projection optics of an EUV lithography system. Condensed outgases absorb EUV light and with time significantly reduce the total reflectivity of the projection optics of an EUV lithography system. Mitigating photoresist outgassing therefore is extremely important in an in-vacuum EUV lithography system having projection optics between the mask and the wafer to be exposed. If photoresist outgasing is not controlled or mitigated in such an EUV lithography system, outgases will render the EUV lithography system useless in a very short time (i.e., in about 100 seconds).
In order to preclude photoresist outgased byproducts from condensing on the projection optics of an EUV lithography system, the wafer stage of a EUV lithography system must be housed in a separate chamber from the projection optics. Theoretically, the wafer stage chamber of an EUV lithography system could be connected to the projection optics chamber by a window, similar to the window of Grobman. A window would allow some light to pass from the projection optics chamber to the wafer stage chamber to expose a coated wafer while preventing photoresist outgases from entering the projection optics chamber and condensing on the projection optics. Using a window, however, would significantly lengthen the minimum time that it takes to reproduce a microelectronic circuit on a semiconductor wafer. This is due to the fact that a window, like condensed outgases, absorbs a significant amount of EUV light, thus lengthening exposure time. Even an extremely thin window would absorb too much light to work with EUV light (i.e., a window, free of outgassing contamination, would absorb more than fifty percent of the incident EUV light). It should be noted here that Grobman is able to use a window only because Grobman uses x-rays, which can penetrate the window without significant losses, to expose the wafer rather than EUV light.
Using a window to prevent outgases from entering the projection optics chamber of an EUV lithography system also has additional drawbacks. For example, outgases would condense and buildup on the window over a short period of time. This buildup of condensed outgases would even further reduce the amount of EUV light that could pass through the window and reach a wafer. Over a short period of time (i.e., less than one hour), the buildup of condensed outgases on the window would reduce the throughput of EUV light to a point where any EUV lithography system (as compared to the x-ray system of Grobman) would be rendered useless.
One windowless means for controlling outgassing in an EUV lithography system is discussed in an article by Jos P. H. Benschop et al., in the September 1999 issue of Solid State Technology, titled xe2x80x9cEUCLIDES: European EUV lithography milestones,xe2x80x9d which is herein incorporated in its entirety by reference. In this article, the authors suggests that by connecting the projection optics chamber and the wafer stage chamber of an EUV lithography system with a tube, and injecting a gas into the connecting tube, a gas flow can be established from the tube into the wafer stage chamber that will apparently preclude photoresist outgases from entering the projection optics chamber. Apparently, this device is based on the idea that outgases will not travel against the gas flow that the authors suggest can be established from the connecting tube into the wafer stage chamber.
While the photoresist outgassing control means suggested by Jos P. H. Benschop et al might work in some system, it will not work in EUV lithography systems that use positional monitoring devices to keep a wafer in focus during exposure. Positional monitoring devices of the type known to those skilled in the relevant art, for example, very accurate capacitance focusing devices or gages that use changes in the capacitance of a device to detect small changes in the position of a surface near the device, must be mounted on a stable surface that is in close proximity to the wafer (i.e., these devices must be mounted on a stable surface close to the wafer so that the end of the device is firmly held within about one millimeter of the wafer). The most stable surface available for mounting positional monitoring devices is the partition located between the projection optics chamber and the wafer stage chamber, and thus the partition is the best place for mounting the positional monitoring devices. As a result, the wafer must be positioned in close proximity to the partition, and the wafer blocks the flow of gas into the wafer stage chamber from the connecting tube discussed by Benschop et al. Most if not all of the gas injected into the connecting tube discussed by Benschop et al. flows into the projection optics chamber rather than the wafer stage chamber because this flow path is the flow path of least resistance.
Therefore, a need exists for a photoresist outgassing mitigation device without a window that will work with any EUV lithography system, including one that uses positional monitoring devices to keep a wafer in focus during its exposure.
The present invention is directed to a photoresist outgassing mitigation system, method, and apparatus. The outgassing mitigation system and apparatus comprise a chimney that is substantially closed at one end, a duct fluidly coupled to the chimney, and a baffle disposed within the chimney. The chimney of the outgassing mitigation apparatus is funnel shaped at the end that is substantially closed. This end of the chimney has an opening that permits a beam or bundle of light to pass through the chimney.
In an embodiment of the present invention, a rotating mechanical barrier, having at least one aperture for the passage of light, is positioned near the chimney so that the rotating barrier substantially closes an open end of the chimney except when one of the apertures of the rotating barrier is passing by the chimney. This rotating barrier is chilled by a refrigerator unit, which is radiantly coupled to a portion of the rotating barrier. A motor having magnetic bearings is used to rotate the barrier. The magnetic bearings thermally isolate the disk from the motor.
In an embodiment of the present invention, a light source synchronisation module is used to trigger a pulsed light source while the apertures of the rotating barrier are aligned with the chimney of the outgassing mitigation apparatus.
In another embodiment of the present invention, the baffle disposed within the chimney is chilled by a cooling unit.
In still another embodiment of the present invention, a barrier gas system is used to inject a barrier gas into the chimney.